1. A 45-year-old man presents to the emergency department with severe headache and vomiting. A CT scan shows a well-circumscribed cystic lesion within the 3rd ventricle; there is no calcium deposition. The cyst is surgically removed. On histologic examination, the wall of the cyst consists of a single layer of mucin-producing columnar epithelium with a ciliated apical surface. Which of the following is the most likely diagnosis?